Architectural effects on fossil preservation. The case of macaroni coralline algae

Abstract

Coralline red algae with protuberances in their thalli are common and instructive examples of fabricational effects on fossil preservation. The body (thallus) of non-geniculate coralline algae is a coherent mass of cell filaments. All vegetative cells, except the epithallial ones at the tip of each filament are enclosed by a high-Mg calcite wall. Many extant and extinct species of coralline algae have protuberances in their thalli. Protuberances appear both on the dorsal surface of algae that grow attached to a rigid substrate and in unattached specimens living on loose sediment. In either case, protuberances develop due to a higher growth rate of fi laments in their centre. In each growth step, the cells formed at the protuberance apex are longer than coeval cells at the protuberance sides. The cell length decreases radially outwards from the protuberance tip. Consequently, central cells in protuberances are generally longer than lateral ones. Calcite walls of central, longer cells are thinner, less dense, and richer in Mg2+ than those of lateral cells. Accordingly, the former are more susceptible to dissolution during fossilization. As a result, dead and fossil coralline algae with protuberance centres partially or totally dissolved are relatively common. In these cases, protuberances occur as structures with an empty or vuggy central part and well-preserved sides. This macaroni-like preservation of corallines is particularly common in Cenozoic marl deposits, formed mainly by concentrations of unattached branching coralline thalli.